1. Field of the Invention
The present invention relates to an improvement of a drive apparatus and a light quantity adjusting apparatus that are suited for a shutter apparatus or the like used in an image pickup apparatus such as a digital camera. The present invention also relates to an improvement of a lens drive apparatus suited to driving a lens of an image pickup apparatus.
2. Related Background Art
A conventional shutter apparatus for use in a lens shutter camera is shown in FIG. 16. In this drawing, reference numeral 101 denotes a magnet that is a permanent magnet, numeral 102 a drive lever, and numeral 102a a drive pin provided for the drive lever 102. This drive lever 102 is fixed to the magnet 101 and rotates integrally with the magnet 101. Reference numeral 103 represents a coil and reference numerals 104 and 105 each indicate a stator that is made of a soft magnetic material and is to be excited by the coil 103. The stator 104 and the stator 105 are connected to each other through portions 104a and 105a and are integrated with each other in terms of a magnetic circuit. When the coil 103 is energized, the stator 104 and the stator 105 are excited and the magnet 101 is driven to rotate within a predetermined angle. Reference numerals 106 and 107 each denote a shutter blade and reference numeral 108 indicates a ground board. The shutter blades 106 and 107 are rotatively attached to pins 108b and 108c of the ground board 108 through hole portions 106a and 107a, with the drive pin 102a being slidably fitted into long holes 106b and 107b of the shutter blades 106 and 107. With this construction, the drive lever 102 rotates along with the magnet 101, the shutter blades 106 and 107 are driven to rotate about the hole portions 106a and 107a, and an opening portion 108a of the ground board is opened/closed. In order to prevent an increase in cost, the magnet is formed using a plastic magnet and the drive pin is formed in an integrated manner.
Reference numeral 109 denotes a front ground board that holds the shutter blades 106 and 107 so as to be movable in a gap between this front ground board 109 and the ground board 108, while reference numeral 110 indicates a rear ground board that holds the stators 104 and 105 and rotatively holds the magnet 101.
By the way, digital cameras are now in widespread use which each photoelectrically convert the image of an objective field using a CCD or the like as an image pickup element and records the image in a storage medium as information of a still image. An example of an exposure operation performed by a digital camera of this type will be briefly described below.
First, the main power supply is turned on prior to photographing and, when an image pickup element is placed in an operable state, shutter blades are maintained at an open position at which exposure of the image pickup element is enabled. Under this condition, the image pickup element repeatedly accumulates, discharges, and transfers electric charges, which makes it possible to observe the objective field using an image monitor.
Following this, when a release button is depressed, a diaphragm value and an exposure time are determined in accordance with an output of the image pickup element at this point in time. When it is judged from the determined diaphragm value and exposure time that the diameter of an exposure aperture needs to be reduced, a diaphragm blade is first driven to a position at which a predetermined diaphragm value is obtained. Next, the image pickup element, from which accumulated electric charges have been discharged, is caused to start accumulation of electric charges. At the same time, an exposure time control circuit is activated in response to the accumulation start signal serving as a trigger signal. Then, when a predetermined exposure time has passed, the shutter blades are driven to a close position at which the exposure light to the image pickup element is blocked. After the exposure light is blocked, the accumulated electric charges are transferred and image information is recorded in a recording medium through an image writing apparatus. The exposure of the image pickup element is inhibited during the transfer of the electric charges because it is required to prevent a situation where the electric charges are changed by extra light during this electric charge transfer.
Aside from the shutter apparatus described above, there are also available a shutter apparatus having a mechanism of moving an ND filter and a shutter apparatus having a mechanism of moving a diaphragm regulating member having a small diaphragm diameter.
With these shutter apparatuses, it becomes possible to realize thickness reduction, although a large area of a ground board is occupied by a coil and a stator. In view of this problem, there is proposed a light quantity adjusting apparatus shown in FIG. 17.
In this drawing, reference numeral 201 denotes a tubular rotor, with its portion 201a being magnetized to have the N pole and its portion 201b being magnetized to have the S pole. Reference numeral 201c represents an arm formed integrally with the rotor 201, and reference numeral 201d indicates a drive pin extending from this arm 201c in a rotation axis direction of the rotor 201. Reference numeral 202 indicates a coil that is disposed in the axis direction of the rotor 201. Reference numeral 203 denotes a stator that is made of a soft magnetic material and is to be excited by the coil 202. This stator 203 includes an outer magnetic pole portion 203a opposing the outer peripheral surface of the rotor 201 and an inner cylinder inserted into the rotor 201. Reference numeral 204 indicates an auxiliary stator that is fixed to the inner cylinder of the stator 203 and opposes the inner peripheral surface of the rotor 201. When the coil 202 is energized, the outer magnetic pole portion 203a and the auxiliary stator 204 are excited and the rotor 201 rotates to a predetermined position. Reference numerals 207 and 208 each represent a shutter blade, while reference numeral 205 indicates a ground board. The shutter blades 207 and 208 are rotatively attached to pins 205b and 205c of the ground board 205 through hole portions 207a and 208a, with the drive pin 201d being slidably fitted into long holes 207b and 208b. Reference numeral 206 denotes a torsion spring that gives an elastic force to the rotor 201 so that the drive pin 201d is pressed against end portions of the long holes 207b and 208b. When the coil 202 is energized, the drive pin 201d rotates along with the rotor 201 in defiance of the elastic force given by the torsion spring 206, so that the shutter blades 207 and 208 are driven to rotate about the hole portions 207a and 208a and an opening portion 5a of the ground board is opened/closed.
With such a construction, it becomes possible to construct a compact light quantity adjusting apparatus.
The light quantity adjusting apparatus shown in FIG. 17 has a shape suited for miniaturization, in comparison with the apparatus shown in FIG. 16. However, the energization needs to be continued when it is desired to move the shutter blades to a close position in defiance of the elastic force given by the torsion spring and to maintain the shutter blades at the close position. Also, in order to maintain the shutter blades at the open position and the close position without using the torsion spring, there may be utilized the direction of energization of the coil. Even in this case, however, in order to maintain the drive target member at a position at which this member is abutted against an end portion of the long hole of the ground board, it is required to continue the energization of the coil. Accordingly, there still remains room for improvement by which power saving is achieved.
According to one aspect of the present invention, a drive apparatus for maintaining a rotor at two stoppage positions includes: a magnet portion having a cylindrical shape whose outer peripheral surface is circumferentially divided into sections that are magnetized to have different poles; a rotor that is capable of rotating about an axis; at least one outer magnetic pole portion that is formed so as to protrude and extend parallel to the axis and opposes the outer peripheral surface of the magnet portion; an inner magnetic pole portion disposed at a position opposing the outer magnetic pole portion so as to oppose an inner peripheral surface of the magnet portion; and a coil that is disposed in an axis direction of the rotor and excites the outer magnetic pole portion and the inner magnetic pole portion. In the drive apparatus, when a value of a ratio of a central angle of the outer magnetic pole portion to a central angle of each magnetized pole of the magnet portion is referred to as Y, and a value of a ratio of a thickness of the magnet portion in a diameter direction to a circumferential length of each magnetized pole of the magnet portion is referred to as X, the following condition is satisfied:
xe2x88x920.3X+0.63 greater than Y. 
According to another aspect of the invention, a light quantity adjusting apparatus for maintaining a light quantity adjusting member at two stoppage positions includes: a magnet portion having a cylindrical shape whose outer peripheral surface is circumferentially divided into sections that are magnetized to have different poles; a rotor that is capable of rotating about an axis; an output member that operates in accordance with rotation of the rotor; at least one outer magnetic pole portion that is formed so as to protrude and extend parallel to the axis and opposes the outer peripheral surface of the magnet portion; an inner magnetic pole portion disposed at a position opposing the outer magnetic pole portion so as to oppose an inner peripheral surface of the magnet portion; a coil that is disposed in an axis direction of the rotor and excites the outer magnetic pole portion and the inner magnetic pole portion; a ground board provided with an opening portion; and a light quantity adjusting member that is driven by the output member so as to move between a position closing the opening portion of the ground board and a position away from the opening portion, and changes a light quantity passing through the opening portion. In the light quantity adjusting apparatus, when a value of a ratio of a central angle of the outer magnetic pole portion to a central angle of each magnetized pole of the magnet portion is referred to as Y, and a value of a ratio of a thickness of the magnet portion in a diameter direction to a circumferential length of each magnetized pole of the magnet portion is referred to as X, the following condition is satisfied:
xe2x88x920.3X+0.63 greater than Y. 
According to yet another aspect of the present invention, a lens drive apparatus for maintaining a lens at two stoppage positions includes: a magnet portion having a cylindrical shape whose outer peripheral surface is circumferentially divided into sections that are magnetized to have different poles; a rotor that is capable of rotating about an axis; an output member that operates in accordance with rotation of the rotor; at least one outer magnetic pole portion that is formed so as to protrude and extend parallel to the axis and opposes the outer peripheral surface of the magnet portion; an inner magnetic pole portion disposed at a position opposing the outer magnetic pole portion so as to oppose an inner peripheral surface of the magnet portion; a coil that is disposed in an axis direction of the rotor and excites the outer magnetic pole portion and the inner magnetic pole portion; a ground board provided with an opening portion; and a lens that is driven by the output member so as to move between a position closing the opening portion of the ground board and a position away from the opening portion, and changes a focal length of a luminous flux passing through the opening portion. In the lens drive apparatus, when a value of a ratio of a central angle of the outer magnetic pole portion to a central angle of each magnetized pole of the magnet portion is referred to as Y, and a value of a ratio of a thickness of the magnet portion in a diameter direction to a circumferential length of each magnetized pole of the magnet portion is referred to as X, the following condition is satisfied:
xe2x88x920.3X+0.63 greater than Y. 
With these constructions, the rotor is stably maintained at predetermined two positions by a magnetic force acting between the magnet portion and the outer magnetic pole portion. Once the coil is energized and the rotor is rotated to either of the two positions, even if the energization of the coil is terminated, the rotor is maintained at the rotation position, which makes it possible to perform setting of the rotor at the two positions with less power consumption.
Other objects and advantages besides those discussed above shall be apparent to those skilled in the art from the description of a preferred embodiment of the invention which follows. In the description, reference is made to accompanying drawings, which form a part hereof, and which illustrate an example of the invention. Such example, however, is not exhaustive of the various embodiments of the invention, and therefore reference is made to the claims which follow the description for determining the scope of the invention.